Ketones with various chain lengths
are widely found in nature where they contribute to the flavors and odors in
animals as well as in plants. Some short-chain ketones are responsible for
mushroom odor, such as 3-octanone and 1-octen-3-one (Maga
JA, J Agric Food Chem 1981, 29, 1). 3-Octanone is a volatile infochemical present in fungi and
recognisable by fungivores (Holighaus G et al., Chemoecology 2014, 24, 57). The 2-decanone is
concentrated in the oily extract of a Jamaican sponge, Plakortis sp. (Bowling
JJ, et al., Chemoecology 2010, 20, 207). It strongly inhibited attachment of
bacteria and mussels. This antifouling property may be used in improving this
product into coating design. 2-Heptadecanone is found in the web of some spiders
(Pholcidae) to attract females (Schulz S, J Chem Ecol 2013, 39, 1). Two
alkyl ketones were shown to be released as pheromones by adult female spiders of
the family Agnelidae : 6-methyl-3-heptanone and 8-methyl-2-nonanone (Schulz
S, J Chem Ecol 2013, 39, 1).
As a first example, several 2-methyl ketones, with a 9 to 15 carbon-chain
length, were identified in milk and milk products. Some of them are also present
in brandies, where they are produced through b-oxidation
and decarboxylation of fatty acids by yeast. One of 2-methyl ketones,
2-tridecanone, is secreted by the glandular trichomes in the tomato leaves and
as an insecticide makes the plant resistant to a variety of insects.
A hydroxylated alkyl ketone, named CAI-1, 3-hydroxytridecan-4-one, was
determined to be the main auto-inducer (quorum sensing substance) in Vibrio
cholerae (Higgins DA et al., Nature 2007, 450, 883). At high cell
density, the presence of that auto-inducer represses both the expression of
virulence factors and the formation of biofilms. Thus, these findings suggest
that this signaling molecule could be used as a therapy to prevent cholera
infection.

3-Hydroxytridecan-4-one (CAI-1)

Several unsaturated ketones with one
or two triple bonds (ketoalkenynes) have been identified in lipid extracts of
Echinacea (Barnes J et al., J Pharm Pharmacol 2005, 57, 929). With some
alkylamides, these compounds are at the basis of the use of these plants to
treat infections, to aid wound healing and to enhance the immune system. Among
these ketones, the most active has 15 carbon atoms one double and two triple
bonds (pentadeca-8-ene-11,13-diyn-2-one) and was determined to be responsible for inhibitory effects on NO and PGE2
production by macrophages (Zhang
X et al., Phytochemistry 2012, 74, 146). One of the acetylenic
ketones extracted from E. pallida, pentadeca-8,13-dien-11-yn-2-one is the
most cytotoxic when tested on human cancer cell lines (Chicca
A. et al., Brit J Pharmacol 2008, 153, 879).

Pentadeca-8-ene-11,13-diyn-2-one

Other similar but less active compounds, some of them being hydroxylated, are
present in lipid extracts of Echinacea.

Symmetrical ketones.

They are commonly found in epicuticular wax
at the surface of many angiosperms. They have the common formula :

[CH3(CH2)n]2C=O
with n = 3-20

One of the most common of these
symmetrical ketones is palmitone (with n = 14).

Cyclic ketones.

These compounds are known to have sex pheromone activities in mammals,
such as civetone (1-cycloheptadecen-10-one) and muscone
(1-methyl-cyclopentadecan-3-one).

Furthermore, it was shown that the
odor of cyclic ketones in related to the ring size.

2-Ethyl-5-propylcyclohexan-1,3-dione (chiloglottone1) is produced by orchids
which employ sexual deceit to attract males of their pollinator species (Frank S
et al., PNAS 2009, 106, 8877). This specific volatile signal mimics
female-released sex pheromones and proved to be important in the relations
between orchids of the genus Chiloglottis, native to Australia, and their
pollinator species.

Chiloglottone1

In 1972, LeTellier et al. reported
that a group of compounds known as 2-alkylcyclobutanones is formed from triacylglycerols
on irradiation and are found in treated foods (LeTellier PR et al., Lipids
1972,7, 75). Indeed, it has been demonstrated that treatment of food products
with a low dose of ionising radiation followed by suitable storage will reduce
bacterial loads and increase shelf-life of the products. There is a debate on
the health risks related to the consumption of irradiated foods containing.
These 2-alkylcyclobutanone compounds have the same number of carbon atoms as
their precursor fatty acids and the cyclobutanones formed from palmitic, stearic,
oleic and linoleic acids are 2-dodecyl-, 2-tetradecyl-, 2-tetradecenyl- and
2-tetra-decadienylcyclobutanone, respectively.

2-Alkylcyclobutanones

The evidence of the natural existence of 2-tetradecenylcyclobutanone and 2-tetradecylcyclobutanone
in non-irradiated cashew nut and 2-decylcyclobutanone in nutmeg was demonstrated,
thus disproving the hypothesis that these compounds are unique radiolytic products
(Variyar
PS et al., J Agric Food Chem 2008, 56, 11817).
A method for the detection of irradiation treatment published by the European
Committee for Standardization, consists in the determination of 2-alkylcyclobutanones,
but it may only be applied with complete confidence to foodstuffs treated for
the purpose of microbial disinfection (doses >0.5 kGy), whose fat content
is higher than 1.0 g % (Ndiaye B et al., Rad Phys Chem 1999, 55, 437).

Long-chain ketones

Formation of long-chain ketones containing from 29 to 35 carbon atoms are
formed by pyrolysis of free fatty acids or triacylglycerols.
Experimentation involving heating of oleic acid and palmitic or stearic acid
at temperature higher than 300°C provided evidence of the formation of mixtures
of ketones with 33 or 35 carbon atoms (Evershed RP et al., Tetrahedron Lett
1995, 36, 8875).

n = 14 or 16

These compounds were studied to
determine the origin of organic residues preserved in archaeological pottery but
they may originate from both animal tissues or higher plant leaf waxes.Alkenones.

These long-chain unsaturated methyl or ethyl ketones are very important compounds which are
produced by a specific class of phytoplankton represented notably by the
coccolithophorid Emiliana huxleyi (Volkman JK et al., Phytochemistry
1980, 19, 2619), the most abundant unicellular phytoplankton which plays a fundamental role in
the total primary production in the oceans. Satellite
images clearly demonstrate that point. The Emiliana blooms have a
major impact on the biological carbon cycle in the ocean and on ocean/atmosphere
fluxes of carbon dioxide (Westbroek et al., Global Planetary Change 1993, 8,
27).

Emiliana huxleyi

Alkenones possess several unusual
characteristics, including their very long chain-length (C35-C40) and the
spacing (C7) and configuration (trans) of their positions of unsaturation (Marlowe
IT et al., Chem Geol 1990, 88, 349) and they were shown to be
membrane-unbound lipids (Sawada K et al., Phytochemistry 2004, 65, 1299).
The two most abundant alkenones
(C37:2 and C37:3) have so far unknown physiological function but their
characteristic is that they remain partially intact in oceanic sediments after the
cellular death and thus may be used as biomarkers.

Until now they were reported
exclusively from the oceanic haptophytes Emiliana huxleyi and Gephyrocapsa
oceanica and from coastal species of Chrysotila (Rontani JF et al.,
Phytochemistry 2004, 65, 117).
As for the phospholipid fatty acids in all living cells, the proportion of the
more unsaturated alkenone is increased when the growth temperatures get colder
and vice versa. Thus, a simple index ([37:2]/{[37:2]+[37:3]}) was formulated to
quantify the degree of unsaturation in a given alkenone series and shown to be
linear versus the growth temperature (Prahl FG et
al., Nature 1987, 330, 367). Consequently, stratigraphic measurement of that
index in dated sediments is done by paleoceanographers to assess climate changes
on timescales ranging from interannual (El Nino) to millenial
(glacial/interglacial). Thus, the variations in sea-surface of the eastern
equatorial Atlantic over the past 500,000 years were inferred from the
distribution of alkenones in sediments (Brassell SC et al., Nature 1986, 320,
129).
Similarly, the dissolved carbon dioxide concentration of surface waters in which
E. huxleyi grew can be calculated using the proportion of the two
different isotopic forms, 12C and 13C.
Thus, for their peculiarities, alkenones are not only the most abundant
extractable lipids in Quaternary marine sediments but also the most precious
molecules for the knowledge of past sea-surface temperature.